"Saw your recent blog post and thought you may get a kick out of a little photoshop tweak if you haven’t stumbled across it yet. Under 'View -- Proof Setup -- Custom' you can set the device to simulate to Gray Gamma 1.8. Now, whenever you hit 'cntrl+y' you get a realtime look at your values without the need to make any conversions. Hit 'cntrl+y' again and you’re back to color. You can also make adjustments in this mode."

Yes I learnt about it recently too.A colleague of mine went even further, showing me what he usually does to tweak values.If you convert to Lab mode and display only the Lightness channel, you have a perfect conversion to grey scale (you basically hide all colour information).You can then paint in B&W on the lightness channel without messing up with hue and saturation.